plants acquired photosynthetic bacteria as endosymbionts (now known as chloroplasts), again conferring on them the ability to harness energy from the sun through photosynthesis. Some plants went even further and also recruited into their root cells prokaryotic endosymbionts (both bacterial and archaeal), which could fix atmospheric nitrogen into nitrogenous compounds that the plant could then use as fertilizer.
As animals and eventually people emerged, bacteria were quick to take advantage of the warm, moist environment offered by their intestinal tracts. In turn, many of these bacteria helped to provide nutrients for the animal or human host by metabolizing the intestinal contents. There is no point in a human life span, except for the brief time the fetus spends in the uterus, during which the human body is not heavily colonized by large numbers of bacteria, especially on the skin, in the mouth, in the intestinal tract, and in the vaginal tract. These bacteria are highly adapted to their niches in and on the human body. However, their constant presence puts them in a position to take advantage of any breach in the defenses protecting the interior tissues and bloodstream from bacterial invasion.
Although some eukaryotic microbes such as algae have a photosynthetic lifestyle, many others, especially amoebas and other protozoa, live by feeding on bacteria and archaea. An interesting aspect of this protozoal grazing that has attracted particular attention recently is that protozoa have properties that are remarkably similar to human phagocytic cells, cells that form an important part of the defenses of the human body. Some of these human phagocytic cells function mainly to engulf, break down, and clear bacteria in blood and tissues. Of these, some then present components of broken-down bacteria to the cells of the immune system.
When animals and humans finally appeared on the evolutionary scene, bacteria immediately took advantage of them as rich niches in which to grow. To a bacterium accustomed to the vagaries of the external environment, where ambient temperature and availability of water and nutrients can vary widely (and unpredictably), a warm-blooded animal whose body temperature is stably maintained and whose lifestyle involves constantly collecting food and water from the environment must be as close as it gets to bacterial heaven. As such, it should not be surprising that the bodies of humans and animals carry dense bacterial loads, especially in the mouth, intestinal tract, and vaginal tract. Small wonder that the human or animal body is often referred to in the scientific literature as the host and that the interaction between a bacterium and its host is referred to as a host-microbe relationship.
Until recently, scientists studying the evolution of insects, animals, and humans almost completely ignored the selective pressure exerted by the long-term presence of the large and diverse populations of bacteria with their hosts. Now, a rapidly expanding area of research has emerged that is devoted to studying the coevolution of hosts with their resident microbial communities, also known as microbiomes. As will become evident in the next few chapters, the effects of microbial pressure can be seen clearly in the design of human skin, eyes, lungs, intestinal tract, vaginal tract, and in particular the immune system. Overwhelming evidence points to the vital role that microbiomes play in both promoting health and modulating disease susceptibility and severity. For the first time, we are also realizing the importance of considering the ancestry of both the host and the microbe when considering pathogenic potential (Box 1-1).
Box 1-1.
Sharing an Ancestral Relationship with Your Resident Pathogen Can Prevent Stomach Ulcers and Cancer
After more than two decades of controversy and intense experimental investigation, it is now well-established that Helicobacter pylori, a bacterium that colonizes the stomach of nearly half of the world’s population, is the leading cause of gastric inflammation, which can lead to stomach ulcers and in a small percentage of infected individuals (<1%) stomach cancer. Extensive phylogenetic evidence suggests that H. pylori bacteria are about as old as modern humans and that since leaving Africa with their hosts these bacteria have diversified geographically in parallel with their human hosts. But until recently, researchers had a hard time correlating the prevalence of H. pylori infections with the incidence of cancer.
The first clue that other factors were at play came from comparing disease prevalence in two populations of Colombians, located about 200 kilometers from each other, with similar levels of H. pylori infection. In this study, the researchers compared H. pylori strains colonizing a coastal population of largely African ancestry (58%) having a low incidence of gastric cancer (∼6 per 100,000) with those from a mountain population of mostly Amerindian descent (68%), in which gastric cancer is more common (∼150 per 100,000). The results showed that Colombians of African ancestry infected with African bacterial strains had low incidence of disease, while Colombians of Amerindian ancestry infected with African strains had a high risk of cancer. Indeed, the more Amerindian ancestry an individual had and the more African-like the H. pylori strains harbored by that individual were, the more likely that person was to have severe gastric disease.
So, it looks like the longer you have coevolved with your resident pathogen, the less likely you are to progress to a diseased state. These findings further support a popular theory of pathogenesis that chronic pathogens that spread through vertical transmission from parent to child are predicted to become less virulent over time as a consequence of coevolution.
Sources:
Moodley Y, Linz B, Bond RP, Nieuwoudt M, Soodyall H, Schlebusch CM, Bernhöft S, Hale J, Suerbaum S, Mugisha L, van der Merwe SW, Achtman M. 2012. Age of the association between Helicobacter pylori and man. PLoS Pathog 8:e1002693.[PubMed][CrossRef]
Kodaman N, Pazos A, Schneider BG, Piazuelo MB, Mera R, Sobota RS, Sicinschi LA, Shaffer CL, Romero-Gallo J, de Sablet T, Harder RH, Bravo LE, Peek RM Jr, Wilson KT, Cover TL, Williams SM, Correa P. 2014. Human and Helicobacter pylori coevolution shapes the risk of gastric disease. Proc Natl Acad Sci USA 111:1455–1460.[PubMed][CrossRef]
Pressing Current Infectious Disease Issues
Having made this digression into ancient history, let us now return to the present and examine some bacterial infections that are at the forefront of burning public health issues. These include emerging infectious diseases, increasing problems with large outbreaks of foodborne and waterborne infections, hospital-acquired (nosocomial) infections, disease transmission, antibiotic resistance, microbiota shift diseases, pathogen evolution, and bioterrorism.
Emerging and Reemerging Infectious Diseases
The emergence of apparently new bacterial diseases and the reemergence of old diseases thought to be under control (at least in developed countries) was an unpleasant shock to the health care community. Emerging and reemerging infectious diseases illustrate an important principle. Disease patterns change, both because bacteria change by acquiring new traits through genetic mutation and horizontal gene transfer and because changing human activities can create new opportunities for bacteria to cause disease.
Not all diseases are truly emerging in the sense of being completely new to the human population. In some cases, the disease symptoms have been around for a long time as a known disease, but the bacterial cause has only recently been identified. A good example of this phenomenon is gastric ulcers, which are now known to be caused largely by the bacterium Helicobacter pylori. This bacterium eluded notice previously because the methods for cultivating and identifying it had not yet become common practice and because many medical researchers were convinced that no bacteria could colonize the human stomach. In this sense, many of these diseases are “emerging” in terms of public awareness but not in the minds of the scientists
